1. Field of the Invention
The invention relates to a composite amplifier and in particular to a multi push-pull composite MMIC amplifier having simplified impedance matching.
2. Description of the Prior Art
The cost of gallium arsenide GaAs monolithic microwave integrated circuits (MMICs) is directly related to the yield and area of the individual MMIC chips. Lower yield and larger chip area mean fewer good chips per wafer and accordingly result in higher chip cost. MMIC power amplifiers employ large periphery power field effect transistors (FETs). These devices greatly effect the cost of the chip due to the necessity for employing complicated matching and combining circuitry which requires a large chip area. Processing technologies, such as E-beam direct-write gates are currently being used to improve the FET yields. However, the large chip area requirement has remained a problem.
Some approaches such as described by Avasarala et al , "A 2.5 Watt High Efficiency X-band Power MMIC" 1989, IEEE Monolithic Circuits Symposium Digest, pp. 25-28, June 1989 describes a partial matching approach. In the arrangement described, certain critical matching circuitry is placed on the MMIC chip, while the remainder the circuitry is implemented in a hybrid environment. The MMIC chip area is therefore reduced. Unfortunately, the reduced chip cost is substantially offset by the increased cost of assembly in post-production tuning of the circuit. In addition, partially matched power MMICs cannot, in general, be RF tested on the wafer as part of the usual screening procedure.
In a patent R.G. Freitag et al. U.S. Pat. No. 4,547,745 entitled "Composite Amplifier With Divider/Combiner" assigned to Westinghouse Electric Corporation, the assignee herein, a MMIC power combiner is described. A large periphery FET is subdivided into a plurality of smaller MESFETs which are coupled between impedance matched input and output transmission lines. Matching is achieved by means of inductive and capacitive elements which are formed on the chip. Some capacitive elements require a connection to ground through a via formed in the chip. Also the capacitors are typically low Q devices which diminish gain and bandwidth. Large bypass capacitors are also susceptible to failure and can negatively effect yield as one bad capacitor will ruin the entire chip.